Efforts to reduce dependence on fossil fuels for transportation fuel and as feedstock for industrial chemicals have been undertaken for decades, with a particular focus on enabling economic feasibility of renewable feedstock. Heightened efforts are being made to more effectively utilize renewable resources and develop “green” technologies, due to continued long-term increases in the price of fuel, increased environmental concerns, continued issues of geopolitical stability, and renewed concerns for the ultimate depletion of fossil fuels.
Conventional biofuel production from renewable feedstock employs a two-step process. In the first step, fermentable sugars are produced from biomass, typically by enzymatic saccharification. In the second step, the sugars are fermented into biofuels or chemicals. This two-step process, however, presents several technical challenges.
For example, biomass needs to be pretreated before hydrolysis can take place to produce sugars. Digestibility of cellulose in biomass is hindered by various physicochemical, structural and compositional factors. Pretreating biomass helps digest the cellulose and hemicellulose fractions of biomass by breaking down the lignin structure and disrupting the crystalline structures of cellulose and hemicellulose. This makes the biomass more accessible to hydrolysis for producing sugars used in subsequent fermentation. Common pretreatments known in the art involve, for example, mechanical treatment (e.g., shredding, pulverizing, grinding), concentrated acid, dilute acid, SO2, alkali, hydrogen peroxide, wet-oxidation, steam explosion, ammonia fiber explosion (AFEX), supercritical CO2 explosion, liquid hot water, and organic solvent treatments. These pretreatment options, however, are often expensive and technically difficult to implement on a commercial scale.
Moreover, acid conversion of biomass to produce the sugars often encounter mass transfer limitations that may reduce overall reaction yields and limit control of product selectivity. Grinding may improve mass transfer rates by reducing particle size; however, for solution-phase systems, the biomass particle size may be approximately one micron or less before mass transfer is no longer rate-limiting. Grinding biomass to this particle size may often be energy-intensive and commercially impractical.
Solution-phase hydrochloric acid (HCl)-catalyzed hydrolysis of cellulose may offer high glucose yields, but commercialization has been challenging. Technical considerations that may be expensive to address on a commercial scale include the high concentrations of aqueous HCl (≧40%) used for effective conversion and throughput under conditions of moderate temperature hydrolysis; high energy requirements for HCl recycling due to formation of boiling azeotropes of HCl and water at concentrations of 20 wt %; additional energy requirements to recover HCl solvent from the slurry cake formed from lignin that is saturated with HCl-rich solutions; and the use of large glass-lined reactors, which are often expensive, due to the high corrosiveness of HCl.
Once biomass is hydrolyzed to form sugars, challenges also exist to purify the resulting sugars and to remove hydrolysis by-products (e.g., acetate and formate). For example, if the cellulose used as a starting material is not pure, the sugars produced may be harder to isolate.
Substituted furans (e.g., halomethylfurfural, hydroxymethylfurfural, and furfural) produced from biomass may be converted into furanic derivatives used as biofuels and diesel additives, as well as a broad range of chemicals and plastic materials. For example, 5-(chloromethyl)furfural can be converted into 2,5-dimethylfuran, which may be used as a biofuel. Additionally, 5-(chloromethyl)furfural can be converted into 5-(ethoxymethyl)furfural, which is a combustible material that may be used as a diesel additive or kerosene-like fuel. Furanic derivatives, however, are currently underutilized to produce chemical commodities because the commercial production methods are not economical.
The production of 5-(chloromethyl)furfural and 5-(hydroxymethyl)furfural from cellulose was first described in the early 1900s; however, slow kinetics and harsh reaction conditions make this method of biofuel production commercially unattractive.
What is needed in the art is a method to directly prepare biofuels and chemicals from biomass, thereby addressing some of the challenges associated with the conventional two-step process involving enzymatic saccharification and fermentation. What is also needed in the art is a method to prepare substituted furans (e.g., halomethylfurfural, hydroxymethylfurfural, and furfural) from biomass containing cellulose and/or hemicellulose in an efficient and cost-effective way. Once these substituted furans are produced, they can serves as intermediates that can be converted into to furanic derivatives such as biofuels, diesel additives, and plastics.